Students get their results at Parrs Wood Sixth Form in Didsbury, Manchester. The UK is 19th on a list of European countries ranked by adults with a minimum of GCSE-level education. Photograph: Christopher Thomond for the Guardian

Several former eastern bloc countries now have adult populations that are more highly educated than the UK’s, the Eurostat data reveals. They include Hungary, the Czech Republic, Poland and Bulgaria.

Statisticians ranked 33 countries according to the percentage of their adult population aged 25 to 64 who had completed, at minimum, upper secondary school – the equivalent to GCSEs – in 2010.

The UK was 19th, with almost a quarter of adults (24%) not having GCSEs or the equivalent. Lithuania came top with only 8% of adults failing to complete the equivalent of GCSE courses or vocational and technical courses at the same level. Turkey was bottom, with 72% of its adults without GCSE equivalent.

Former Communist countries such as Poland (11%) and Bulgaria (21%) outperformed the UK. On average across the 33 countries, 27% of adults had not completed sixth-form study.

The Eurostat percentages encompass each national population’s educational attainment ranging from GCSE equivalent, at minimum (defined in detail under the terms of ISCED level 3 – the international standard classification of education), and upwards, to PhD level at maximum (ISCED level 6).

The lecturers’ union, the University and College Union, said the figures showed the UK was languishing in “mid-table obscurity”.

Sally Hunt, the union’s general secretary, said there was a “very real possibility” that coalition reforms could lead to the country sliding further down the table in future years. She said the near-trebling of university tuition fees to up to £9,000 a year and restrictions on university places would have a detrimental effect on the nation’s qualifications.

However, a spokeswoman from the Department for Business, Innovation and Skills (BIS), whose remit includes universities, said the coalition was overhauling the school system to ensure the poorest could study at college and university and creating thousands more higher-level apprenticeships.

Last month, ministers said there would be fewer university places at English universities this autumn. In previous years, an extra 10,000 places had been created to accommodate demand, but these will not be available this year. Some 5,000 places for universities that over-recruit have also been taken away.

The BIS spokeswoman said the number of full-time undergraduates in 2012-13 would remain at record levels.

In December 2010, a study of 65 countries showed the UK had slipped down world education rankings in maths, reading and science, and had been overtaken by Poland and Norway. The study, compiled by the Paris-based Organisation for Economic Co-operation and Development, revealed that the UK’s reputation as one of the world’s best for education was at risk.

• This article was corrected on 14 February 2012. The original heading (UK trails Poland and Bulgaria on adults educated to A-level standard) and the story text said that the Eurostat data were comparing what proportion of various nations’ populations had completed the equivalent of English A-level exams. This has been corrected, because the Eurostat table’s reference to completion of at least “upper secondary school” means GCSE or equivalent; the Eurostat figures cover the proportion of people in countries surveyed who have completed anything from GCSE level right up to PhD level. An explanatory note to this effect has been inserted in the text.